1. Field of the Invention
The present invention relates to a valve system for an internal combustion engine, which is capable of opening and closing intake valves and exhaust valves of the internal combustion engine at different timings according to operating states of the internal combustion engine.
2. Description of the Related Art
In recent years, a valve system (also referred to as a xe2x80x9cvariable valve systemxe2x80x9d), which is capable of changing operating characteristics (such as opening/closing timing, opening time, and so forth) of intake valves and exhaust valves (hereinafter generically referred to as xe2x80x9cengine valvesxe2x80x9d or xe2x80x9cvalvesxe2x80x9d) provided in a reciprocating internal combustion engine (hereinafter referred to as xe2x80x9cenginexe2x80x9d) to the optimum characteristics according to engine load and speed, have been developed and put into practical use.
As an example of a mechanism for changing operating characteristics as described above, a variable valve system is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-41017. This variable valve system is comprised of a connection mechanism constructed such that a low-speed rocker arm is provided with a hydraulic piston, and an engagement projection formed in a high-speed rocker arm is selectively engaged with the piston such that the valve timing can be switched between a low speed and a high speed.
This variable valve system is also provided with a return spring that forces the piston toward the bottom of a cylinder. In the state in which oil pressure is not applied to the piston, the piston is forced by the return spring and is held at a position in the cylinder, where the piston is not engaged with the engagement projection of the high-speed rocker arm.
It is therefore an object of the present invention to provide a valve system for an internal combustion engine, which is capable of switching the timing of an engine valve by a piston provided with a return spring and whose position can be switched, and a member abutted on the piston to move in corporation with the piston, while ensuring a sufficient stiffness of a power transmitting section of the valve system and a sufficient valve lift.
To attain the above object, the present invention provides a valve system for an internal combustion engine that has: a first rocker arm supported by a first rocker shaft such that the first rocker arm rocks with respect to first rocker shaft when driven by a first cam, the first rocker arm having an end thereof connected to one of an intake valve and an exhaust valve; a second rocker arm supported by the first rocker shaft adjacent to the first rocker arm such that the second rocker arm rocks when driven by a second cam having a different cam profile from that of the first cam; a cylinder formed in one of the first and second rocker arms; a piston slidably mounted in the cylinder; an engagement projection provided in a manner being projected from the other one of the first and second rocker arms and is capable of being engaged with an engagement part formed in the piston; and a piston position switching device that switches a position of the piston between an engagement position where the engagement projection is engaged with the piston and a disengagement position where the engagement projection is not engaged with the piston.
With the above arrangement, when the piston position switching devices positions the piston at the engagement position, the engagement projection projected from the other one of the first and second rocker arms is engaged with the engagement part formed in the piston, so that the first and second rocker arm rock integrally with each other to open and close one of the intake valve and the exhaust valve according to the cam profile of the second cam. On the other hand, when the piston position switching devices positions the piston at the disengagement position, the engagement projection projected from the other one of the first and second rocker arms is engaged with the engagement part formed in the piston, so that one of the first and second rocker arm rock independently of each other to open and close one of the intake valve and the exhaust valve according to the cam profile of the first cam.
Further, the piston position switching device is comprised of a return spring that urges the piston towards the disengagement position, and the return spring is disposed in a manner being eccentric from the piston in such a direction as to get away from the engagement projection. With this arrangement, it is possible to easily ensure a sufficient stroke of the engagement projection in such a range as not to interfere with the return spring, and even in the case where the return spring is disposed inside the engagement part formed in the piston, it is possible to easily ensure a sufficient thickness of the engagement part.
As a result, it is possible to enable the engagement projection of the second rocker arm to move in a wide range, and to ensure a sufficient valve lift when the first rocker arm is operated according to the cam profile of the second high-speed cam.